The invention relates to machines for the manipulation of running webs of paper, metallic foil, plastic foil, magnetic tape, textile, laminates and/or other strip-shaped materials. More particularly, the invention relates to improvements in apparatus for thermally influencing selected portions of certain rolls in calenders and like machines. The following description will refer primarily to calenders for the treatment of paper webs; however, the improved apparatus can be put to use with equal or similar advantage in machines wherein pairs or larger groups of rolls are employed to advance, compress, influence the gloss and/or other characteristics of, or otherwise manipulate running webs or strips of flexible material other than paper or including paper and one or more other materials.
Many calenders employ pairs or larger groups of parallel rolls defining nips for running paper webs. The width of a nip is normally or preferably such that the machine can properly manipulate relatively narrow webs, relatively wide webs as well as webs of medium width. As a rule, a running web is guided in such a way that its marginal portions are spaced apart from the respective ends of the peripheral surfaces of the rolls so that a portion of the peripheral surface at each end of the roll is not overlapped by the running web. This can present serious problems in calenders wherein running webs are to be treated by heated rolls, for example, by one pair or by two or more pairs of rolls wherein one roll of each pair is a steel roll (finishing roll) which is heated to an elevated temperature and the other roll of each pair is an elastic roll (pressure roll), i.e., a roll wherein at least a certain layer adjacent the peripheral surface can yield at the nip of the two rolls constituting the respective pair. The elastic roll cannot stand elevated temperatures above a certain value. However, the running web constitutes an effective thermal insulator between the heated roll and that portion of the elastic roll which is overlapped by the running web. Thus, the end portions of an elastic roll are much more likely to be overheated than the median portion which is separated from the heated roll by the running web when the calender is in actual use.
Attempts to prevent overheating of end portions of elastic rolls include the provision of stationary cooling devices at the ends of an elastic roll. Such stationary cooling devices can employ nozzles which discharge jets or streams of compressed air, plenum chambers (boxes) with outlets for compressed air, nozzles which discharge several different coolants, atomizing nozzles and others. Such stationary cooling devices are satisfactory if the width of running webs is constant, i.e., if the width of successively manipulated webs is the same or at least close to an optimum value for a particular setup of cooling devices in the calender. However, the effectiveness of stationary cooling devices is much less satisfactory if the widths of successively manipulated webs are different or if the position of a running web having a satisfactory width changes in a direction toward the one or the other end of the peripheral surface of an elastic roll. This leads to overheating of the one or the other end portion of the peripheral surface of an elastic roll and can cause serious damage to the elastic layer or layers. Thus, if the width of the non-overlapped portion of the peripheral surface at one end of an elastic roll is greatly increased, the respective stationary cooling device is incapable of adequately cooling the exposed end portion of the roll and the elastic layer or layers at the respective end of the elastic roll are destroyed as a result of overheating.
U.S. Pat. No. 3,254,593 granted Jun. 7, 1966 to Beachler discloses a gloss calender drive system and method. The patentee proposes to internally cool elastic pressure rolls with water as well as to cool by a cold air blast those edge portions of the pressure rolls which extend beyond the width of the running sheet or web in order to protect the uninsulated rubber covering of the rolls from the heat radiated by the finishing rolls. In lieu of resorting to such air blasts, the patentee also proposes to taper the outward edges of the pressure rolls outwardly so as to avoid contact between the end portions of pressure rolls and the adjacent portions of the heated finishing rolls. The Beachler reference does not show and/or suggest any particular mounting for the device or devices which are to direct a cold air blast against the edge portions extending beyond the width of the sheet or web.
German patent application Serial No. 39 07 216 A1 of Kalliola (published Sep. 28, 1989) discloses a calender wherein the pressure rolls are protected from overheating by the adjacent finishing rolls in that the width of the running webs exceeds the axial length of the pressure rolls, i.e., the entire peripheral surface of each pressure roll is overlapped by the running web. Kalliola proposes to trim the marginal portions of the web when the treatment is completed, i.e., the width of the web which is to be treated must considerably exceed the required width of the finished web. Such proposal is not entirely satisfactory because it entails much waste in valuable web material.